Conventional radio receivers contain an audio filter which is designed to accomplish a particular objective, Typical objectives include; improving the signal to noise ratio, increasing the intelligibility of the audio output signal when a weak signal is received, or tailoring the audio response to meet certain prescribed listening conditions. It would be virtually impossible to accomplish all of the design objectives in the design of a single audio filter. Tradeoffs would have to be made so as to accomplish the main design objective while optimizing secondary objectives. Quite often the design objectives may conflict. For instance, a filter designed to provide optimal audio fidelity at strong signal levels may result in poor articulation at weak signal levels. Thus, the audio filter incorporated in a conventional radio receiver may be merely designed for a single design objective which is optimized for one particular S/N ratio. In addition, accomplishing multiple objectives may require a filter system with a plurality of frequency response characteristics as opposed to one which merely requires varying the bandwidth. This would be virtually impossible with any single active or passive filter.
There are numerous audio filters in the art which accomplish a single design objective. For example, U.S. Pat. No. 4,484,358 to Iwahashi teaches using an audio filter with a predetermined frequency response characteristic which is comprised of a switched capacitor filter wherein the bandpass of the filter is varied in response to the control signal applied thereto. A signal is picked from the IF section of the receiver. This signal is used to provide a pulse signal for clocking the switched capacitor filter. Varying the frequency of the clock signal will vary the passband of the filter but not the frequency response characteristic. The signal picked up from the IF section is one which has not been amplitude limited. When the amplitude of such signal is below a certain threshold level, the clock pulses are deactivated.
Similar to Iwahashi, U.S. Pat. No. 4,379,207 to Kubota discloses an audio filter for an FM stereo receiver which is controlled by the amplitude of the signal picked up from the IF section of the receiver. In accordance with this invention, depending on the amplitude of the signal from the IF section, high frequency noise is filtered out by way of the audio filter.
U.S. Pat. Nos. 2,293,274, 2,120,998 and 2,136,664 to Barber are examples of adaptive audio filters with a single frequency response characteristic but with a variable passband.
While each of these audio filters is designed to accomplish a single design objective such as improving fidelity, they are not adaptable to accomplishing more than one design objective nor a plurality of frequency response characteristics based upon a plurality of signal strengths.